Q. 30 3.9( 41 Votes )

(i) A lens produces a magnification of -0.5. Is this a converging or diverging lens? If the focal length of the lens is 6 cm, draw a ray diagram showing the image formation in this case.

(ii) A girl was playing with a thin beam of light from a laser torch by directing it from different directions on a convex lens held vertically. She was surprised to see that in a particular direction, the beam of light continues to move along the same direction after passing through the lens. State the reason for her observation. Draw a ray diagram to support your answer.


(i) On entering in a medium from air, the speed of light becomes half of its value in air. Find the refractive index of that medium with respect to air?

(ii) A glass slab made of a material of refractive index n1 is kept in a medium of refractive index n2.

A light ray is incident on the slab. Draw the path of the rays of light emerging from the glass slab, if (i) n1> n2 (ii) n1 = n2 (iii) n1< n2

Answer :

(i) When a convex or converging lens forms a real image, the magnification is considered negative. This is so because the image is inverted.


When the beam of light passes through the optic center, the ray shouldn’t deviate. Here when the girl was playing with the laser beam with convex lens placed vertically, the ray must be passing through the optical center.

The reason being that the middle part of a lens is like a thin glass slab, so almost no deviation takes place and very slight lateral displacement of the incident ray takes place.



We know from Snell’s law; refractive index of a medium is given by-


n = refractive index

c=speed of light in vacuum

v=speed of light in medium

In medium, speed of light becomes 1/2

So refractive index becomes,



Refractive index of slab = n1

Refractive index of medium = n2

(i)When n1> n2, the incident ray bends away from the normal of glass slab as shown in fig. below

(ii) When n1 = n2, there won’t be any refraction

(iii)When n1< n2, the ray of light will bend towards the normal as shown in fig. below.

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